Cannulas for cardiopulmonary bypass systems have thin walls to optimize flow rates through the cannula for a given cannula size. Wall thickness is particularly important when using the system for arresting a patient's heart and placing the patient on cardiopulmonary bypass described in co-pending U.S. patent application Ser. No. 08/570,286, filed Dec. 11, 1995 by Valley et al., which is incorporated herein by reference. Venous blood is withdrawn through a venous cannula and directed to an oxygenator. The oxygenated blood is then returned to the patient's arterial system through an arterial cannula. An aortic occlusion catheter is used to block blood flow through the ascending aorta and deliver cardioplegic fluid to arrest the heart of the patient for performing surgery on the heart and great vessels. The aortic occlusion catheter is inserted through a lumen in the arterial cannula which is the same lumen through which arterial blood is returned.
An advantage of the system described above is that only one opening in the patient's arterial system is required for both delivery of cardioplegic fluid and return of arterial blood. In order to achieve the necessary blood and cardioplegic fluid flow rates, however, the wall of the arterial cannula must be minimized while retaining enough structural integrity to prevent kinking and cracking. The present invention is particularly useful in providing a thin walled cannula which may be used as an arterial return cannula for the system described above.
A known method of making a reinforced cannula is to dip a mandrel in a polymer solution and wrap a metal wire over the polymer. The mandrel is then dipped again to encase the metal wire between two layers of polymer.
Another known method of making a reinforced cannula is to extrude a polymer tubing, wrap a metal wire around the polymer tubing, and extrude another polymer layer over the metal wire.
A problem with the known methods of manufacturing a reinforced cannula is that the spacing between adjacent wires must be relatively large to ensure that the polymer flows between adjacent coils so that the two polymer layers bond together to form an integrated structure. Unfortunately, the relatively large spacing requires a relativley thick polymer layer to provide the necessary strength since the wire has a large pitch. The relatively thick polymer layer is also required to ensure that a sufficient amount of polymer is provided to fill the relatively large space. The resulting cannula has a relatively thick wall.
Thus, a specific object of the present invention is to provide a new method of manufacturing reinforced tubing and, in particular, cannulas for venous withdrawal and arterial return of blood for a cardiopulmonary bypass system.
As mentioned above, an aortic occlusion catheter is passed through the arterial return cannula when practicing the invention disclosed in U.S. patent application Ser. No. 08/570,286. It is also advantageous to minimize the size of the aortic occlusion catheter for the same reasons as it is advantageous to minimize the size of the arterial return cannula. A limiting factor when reducing the size of the aortic occlusion catheter is that the catheter cannot kink when extending around the aortic arch. Thus, another object of the present invention is to provide an aortic occlusion catheter which minimizes the catheter size while providing a catheter which resists kinking.